1. Field of the Invention
This invention relates to a long range artillery shell having a rocket motor.
2. Discussion of Prior Art
There is a constant military requirement to extend the range of artillery shells with minimum effect on the payload and the delivery system. In practice this requires the general physical shape of the conventional shell to be maintained since if the long range artillery shell conforms closely to the mass and external dimensions of the conventional shell it may then be delivered using existing ordnance without exceeding the maximum acceptable breech pressure.
One method to increase the range of a shell is to provide that shell with a rocket motor which is ignited after the round has left the barrel to provide additional impulse to the shell, thereby increasing its range.
Rocket motors are well known for missile systems. For example, UK patent GB676368 discloses a missile with a rocket motor having a propellant arranged in an end burn configuration in a combustion chamber, an axially arranged exhaust nozzle and a number of canted or cambered vanes to interact with the exhaust gases and impart a degree of spin to the missile.
Missiles, however, are generally relatively high cost munitions, a large part of the volume of which is rocket motor, and are launched by ignition of the rocket motor. Ignition takes place before any launch stresses have been encountered and all of the energy imparted to the missile derives from the rocket motor whereas an artillery shell, which is relatively cheap and incorporates a large volume for payload, is fired by a gun and any rocket motor is not ignited until after launch. The launch accelerations experienced by artillery shells (approximately 16,000 g) may be over 200 times greater than those experienced in missile systems. These high axial loads during launch of an artillery shell would cause movement and deformation of the propellant in a rocket motor which could result in failure of the motor to ignite after launch or catastrophic failure of the shell.
Conventionally the rocket motor used in a long range shell comprises a solid propellant configured with a central cavity which extends axially throughout the length of the propellant, forming a plenum chamber. In such a configuration the propellant burns from the cavity outwards to act as a so called internal or perforated grain propellant. The gaseous combustion products produced by this burning then exit the rocket through a nozzle in the rear of the motor casing.
However one problem associated with this type of rocket motor is that the perforated grain propellant may deform when subjected to the high axial acceleration loads experienced during launch. In so deforming the solid propellant may develop a crack or may deform plastically to close up the central cavity. In either case, misfunction of the motor and sometimes catastrophic failure of the shell can result.
One solution to this problem of deformation is to divide the propellant into a series of smaller length grains, along the axis of the shell and separated from one another by supports having a central hole to allow passage of the gaseous combustion products to the nozzle. However, this solution introduces the additional problem that the volume of the combustion chamber available for propellant is decreased by these supports. This results in either a reduced payload or a reduced range if the mass and external dimensions of a conventional shell are to be retained.